1. Field of the Invention
This invention relates to a method of forming a peelable seal for containers. This invention also relates to the shape of the seal structure used to seal a lid to a container.
2. Discussion of the Prior Art
As discussed in U.S. Pat. No. 3,491,935 packages of thermoplastic materials have been used extensively in various industries for dispensing a wide variety of both flowable and non-flowable materials. These packages consist of containers that have been sealed in various ways, and recently there has been a great need for a container sealed with thin-wall sealing material that is either punctured or removed by the consumer to access the contents of the sealed package.
The '935 patent uses a differential method of forming a peelable seal for the containers in which the two thermoplastic materials selected to form the lid and the container have different softening points. These materials are heated while they are in contact with one another so that the first material fuses and secures itself to the second material, and thereby forms the desired peelable seal without fusing the second material (lines 15-22 of col. 2).
In the sealing operation of the '935 patent, the heat flows through the closure material from a heated sealing die which also extrudes a portion of the closure material from beneath the die, so as to form a bead on the flange of the container. It should be noted that the beads formed in this reference are not on the inner rim of the flange.
U.S. Pat. No. 4,519,499 issued to Stone et al on May 28, 1985, (as well as U.S. Pat. No. 4,496,046, same inventor and assignee), discloses a container having a selectively openable seal line and peelable barrier means. The container is made of two sheets which are secured to each other by a seal that includes a bead area of increased thickness at the break end of it, FIGS. 1, 2 and 3; lines 46-48 of col. 2). The sealing die used for sealing the two sheets together consist of a die head and a mirror image die head. Each of them has a convex projection and a bevel surface formed 5 degrees from horizontal FIGS. 7, 8, 9, and 10, lines 15-32 of col. 6). This reference relates to the use of a sealing die having a beveled surface and the formation of beads. However, it does not relate to the formation of beads on the inner rim of a container.
U.S. Pat. No. 3,679,509 issued to Fielibert on July 25, 1972 relates to a process for sealing laminated materials. This reference uses a V-shaped bar as its sealing head in which the obtuse angle ranges form 150.degree.-170.degree. (lines 14-21 of col. 2).
U.S. Pat. No. 4,171,236 discloses a method for forming a fractionable seal of the type wherein a plastic cup-shaped closure is hermetically sealed to a plastic planar surface, such as the neck of pharmaceutical solution containers. The seal is formed with the closure by forcing a die heated above the melting point of the plastic materials into the desired planer surface. Both the planar surface and the closure adjacent thereto become molten and the surface material is displaced to form a fused seal transverse to the surface. Due to the design of the die, a frangible section is formed in the vertical sidewall of the depression. This reference also does not disclose any formation of beads on the inside rim of a container flange.
In food packaging, the area of the invention, is important that the lid remain fixed to the container during distribution of the closed container via various transportation routes to the eventual consumer. In an attempt to standardize the survivability determination of sealed containers, the U.S.D.A. has defined the "Restrained Burst Test" as a means of measuring this ability to maintain package integrity. The restrained burst test involves pressurizing the inside of the subject container with compressed air. The seal container must be able to maintain +5 psig between the inside and outside of the container for at least 60 seconds.
Unfortunately, when one increases the survivability of the package by increasing the width and strength of the seal area between the container and the lid, (such as to withstand a 10 psig pressure differential), the peel strength in the prior art devices also increases in a linear manner such that it becomes very hard for the purchaser of the packaged food container to open the lid. The designer of the food container is therefore faced with a Hobson's choice, wherein a container that may survive the transportation phase will not be openable by the subsequent purchaser, or where an "easy open" container will not survive the transportation process. Increasing the burst strength of a container causes an undesirable increase in the peel strength required to open the container.
Most food packers require a minimum of 10 psi burst pressure and would like 15 psi. With conventional lid sealing, 15 psi burst pressure requires a material that has a peel strength of 10.75 lbs/in. This is extremely difficult to peel.
A method of sealing a lid to a container therefore needs to be developed wherein a high burst pressure for the container does not unduly hamper the consumer from peeling the lid from the container.